Power conversion devices are currently widely used in electronic systems for providing regulated power supplies. There exists a variety of power conversion devices, such as a buck converter, a boost converter, and a flyback converter. Typically, a power conversion device operates under control of a converter controller to obtain a desired output voltage. The converter controller can be powered by a separate switch mode power supply (SMPS) with the benefit of a stable power voltage. However, the SMPS solution requires additional circuitry, and consequently the cost is increased significantly while the system efficiency degrades.
FIG. 1 illustrates a block diagram of a conventional controller 100, which is integrated with a power supply circuit. As an example, a flyback converter 102 is used in FIG. 1 for the purpose of explanation but not limitation. Generally, a flyback converter 102 includes a switch 140, a transformer T1 with a primary winding 103 at the primary side and a secondary winding 105 at the secondary side, a diode 109 and a capacitor 111. When the switch 140 is turned on, a current flowing through the primary winding 103 ramps up and energy from a power source 101 is stored in the core of the transformer T1. During this time interval, the diode 109 is reverse-biased and energy to a load 113 is supplied by the charge in the capacitor 111. When the switch 140 is turned off, the negative current transition on the primary winding 103 is reflected to the secondary winding 105 such that the diode 109 becomes forward-biased and current is conducted to the load 113 and also to recharge the capacitor 111. In general, the flyback converter 102 is an isolated power converter for converting an input voltage VIN to an output voltage VOUT.
The controller 100 is employed to control a conduction status of the switch 140, thereby controlling the output voltage VOUT. Instead of being powered by a separate SMPS, a flyback controller 120 is powered by an auxiliary voltage VAUX derived from the flyback converter 102. The auxiliary voltage VAUX is generated by an auxiliary winding 107. The auxiliary winding 107 may be placed at the secondary side of the transformer T1. As such, the auxiliary winding 107 is magnetically coupled to the transformer T1. The auxiliary winding 107 is further coupled to a diode 115 and a capacitor 117. In a similar way, the secondary winding 105 is coupled to the diode 109 and the capacitor 111. Furthermore, the auxiliary winding 107 and the secondary winding 105 are differently grounded. For example, the secondary winding 105 is coupled to a secondary side ground 121, while the auxiliary winding 107 is coupled to a primary side ground 123. As the switch 140 is turned on and off alternately, the auxiliary voltage VAUX is produced at the secondary side of the transformer T1. Additionally, by adjusting a turn ratio between the primary winding 103 and the auxiliary winding 107, the auxiliary voltage VAUX can achieve a desired voltage level to power the flyback controller 120.
However, the auxiliary voltage VAUX will vary according to the load condition (e.g., a light load condition or a heavy load condition) and thus a large ripple will appear on the power supply of the flyback controller 120. For example, the auxiliary voltage VAUX may vary from 3.5V to 15V. A typical flyback controller needs a minimum power voltage of 6V. As a result, the power voltage (which is herein the auxiliary voltage VAUX) may drop below the minimum supply voltage of 6V required by the flyback controller 120, leading to a power failure. One approach to avoiding such power failure is to increase the number of turns of the auxiliary winding 107 such that the minimum auxiliary voltage VAUX is guaranteed to be greater than the minimum power voltage required by the flyback controller 120 when the auxiliary voltage VAUX varies according to the load condition. However, one drawback of such an approach is that the maximum auxiliary voltage VAUX may exceed the maximum power voltage that the flyback controller 120 can endure.
Therefore, conventional solutions to controller power supply have either cost concerns or stability problems.